I think that when I were in high school if I cared nothing for my grade, I would pay rapt attention in those classes that interested me personally (for me it was math and physics). I would almost completely ignore the other classes and in best case I would spend that class time reading about physics, but more likely daydreaming.
Putting the focus on the subject I think would work quite nicely when it is intinsicly interesting to that student, but for all other subjects I believe there needs to be some form of external motivator.
If the students see grades as important (because their self esteem and pride is at least partially involved for instance, or because they fear not getting into their chosen college) then that can be the external motivator. Otherwise some other reason will need to be fabricated.